When in such a pump device the shaft is driven in rotational direction the rotor means are rotated thereby increasing and decreasing the volume of the pressure chambers. When the volume of the pressure chambers increases liquid is sucked from an inlet and when the volume of the pressure chambers decreases this liquid is outputted through an output. The number of the pressure chambers and the accumulated volume of the pressure chambers define the displacement of the pump means.
The invention relates in particular to a water hydraulic pump device, i.e. a pump device with which water can be pumped and with which the pressure of the water can be considerably increased so that the water can be supplied to a reverse osmosis unit. In this case the water can be purified, for example, to gain drinking water from salt water. In such reverse osmosis applications usually a large amount of water has to be pumped. To this end it is necessary to have a large number of pump devices which makes the whole arrangement expensive. Furthermore, each pump device together with a corresponding driving motor requires a certain space. Therefore, for a high volume of fluid to be pressurized a considerable space is necessary.